Bug#575997: libstdc++6-4.5-dev: c++config.h doesn't seem to match reality

[Miles Bader <miles@gnu.org>]

Matthias Klose <doko@debian.org> writes:
> Same in 4.4.
>
> libstdc++-v3/acinclude.m4:
> dnl Check for clock_gettime, nanosleep and sched_yield, used in the
> dnl implementation of 20.8.5 [time.clock], and 30.2.2 [thread.thread.this]
> dnl in the current C++0x working draft.
> dnl
> dnl --enable-libstdcxx-time
> dnl --enable-libstdcxx-time=yes
> dnl        checks for the availability of monotonic and realtime clocks,
> dnl        nanosleep and sched_yield in libc and libposix4 and, in case, links
> dnl       the latter
> dnl --enable-libstdcxx-time=rt
> dnl        also searches (and, in case, links) librt.  Note that this is
> dnl        not always desirable because, in glibc, for example, in turn it
> dnl        triggers the linking of libpthread too, which activates locking,
> dnl        a large overhead for single-thread programs.
> dnl --enable-libstdcxx-time=no
> dnl --disable-libstdcxx-time
> dnl        disables the checks completely
>
> not enabled by other distributions as well. asked upstream:
> http://gcc.gnu.org/ml/libstdc++/2010-04/msg00007.html

What seems silly is that the interfaces (std::thread::yield,
std::thread::sleep_for) appear to work fine if I simply do:

   #define _GLIBCXX_USE_SCHED_YIELD 1
   #define _GLIBCXX_USE_NANOSLEEP 1

before including <thread>.  The _only_ use of those macros (in the
entire c++ header suite) seems to be in deciding whether to define
the relevant inline method (std::thread::sleep_for, sleep_until,
yield) in the std::thread class.

If those inline methods are never called, then they should have zero
effect (including pulling in any extra libraries etc), so it seems
like defining those macros in c++config.h would similarly have zero
effect if the facilities aren't actually used by the user.

Maybe other (non-linux) systems have other problems, and the gcc
configure script is being a little too careful because of those
other systems?

-Miles

-- 
Bore, n. A person who talks when you wish him to listen.